The present invention relates to a hydraulic control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling a continuously variable transmission having a torque converter with a lockup clutch.
A continuously variable transmission having a torque converter with a lockup clutch is known. The torque converter operates to multiply the torque of an engine, so that the vehicle can be smoothly started. After the vehicle starts, the torque converter is preferably locked up by a lockup system, thereby preventing a loss of power and improving fuel consumption. Thus, various control systems for controlling the lockup clutch have been proposed in order to effectively use such characteristics.
In order to quickly engage the lockup clutch, the lockup system starts to engage the lockup clutch at an early stage where there is still a difference between speeds of the input and the output members of the clutch. Thus, the engine speed suddenly changes, thereby causing a shock.
Accordingly, there has been proposed systems for reducing the shock. Japanese Patent Application Laid-Open 63-192629 discloses a system for controlling a continuously variable transmission wherein the transmission is downshifted at the lockup of the torque converter so that the rotational speed of the output member of the clutch is increased. Thus, the speed difference between the input member and the output member is decreased so that the shock at the lockup is reduced.
However, when the transmission is downshifted, the drive pulley speed, that is the speed of the output member of the clutch is rapidly increased and exceeds the engine speed at the lockup. Hence the engine torque and the acceleration characteristics after the lockup largely change compared with that of before the lockup, which causes an even greater shock. Thus, it is desirable to gradually converge the drive pulley speed to the engine speed at the lockup of the torque converter so that the engine speed and the engine torque may smoothly increase.